Random-laid non-woven fabrics made by a conventional spun bonding method and the like are excellent in bulkiness and texture. The random-laid non-woven fabrics also have excellent water permeability and filtering characteristics. Since, however, filaments are arranged substantially at random in the random-laid non-woven fabrics, the random-laid non-woven fabrics have only a poor dimensional stability and a small strength in the longitudinal and transverse directions. In order to improve the strength, the conventional random-laid non-woven fabrics may be stretched. When the conventional random-laid non-woven fabrics are simply stretched in the longitudinal or transverse direction, however, an interengagement or bondage connecting the filaments is often disconnected, and the filaments themselves are not stretched. Therefore, the strength of the non-woven fabrics is not increased.
In addition, a technique of biaxially stretching normal non-woven fabrics is available as disclosed in British Patent No. 1,213,441. When normal non-woven fabrics are biaxially stretched, however, the efficiency of stretching filaments composing the non-woven fabrics is low. Therefore, the strength of the non-woven fabrics cannot be sufficiently increased.
The present inventors, therefore, proposed a method and apparatus for stretching a non-woven fabric in the longitudinal or transverse direction, a means of cross-laminating a longitudinally stretched non-woven fabric and a transversely stretched non-woven fabric, a means of unidirectionally arranging spun filaments, and the like in Japanese Patent Application No. Sho 62-173927. The invention of this prior application is characterized in that a long-fiber random-laid non-woven fabric obtained by spinning un-oriented filaments is stretched in a predetermined direction so that the filaments composing the non-woven fabric are substantially stretched to cause a molecular orientation. As disclosed in this prior application, when a non-woven fabric in which filaments are unidirectionally arranged is stretched in an arranging direction of the filaments, the filaments are stretched, and the strength of the non-woven fabric is increased. In accordance with the type of filaments composing a non-woven fabric, if only the filaments are arranged, a satisfactory strength or dimensional stability can be obtained in the arranging direction of the filaments without stretching the fabric.
The method of making a sheet by shaking fibers is disclosed in Japanese Patent Publication No. Sho 45-10779. In this method, a fluid is alternately, intermittently sprayed from right and left directions to a portion close to a jet outlet of fibers, thereby shaking the fibers to make a sheet.